Method for molding a ball-like article



` relatively thin neck portion,

United States .Patent METHOD FOR MOLDING A BALL-LIKE ARTICLE Julius I. Sabo and Howard E. Butler, Dayton, Ohio, as-

siguors to General Motors Corporation, Detroit, Mich., a corporation of Delaware v Application July s, 1957, serial No. 669,825

1 claim. (ci. 1s-s9) l This invention relates to a method for making solid articles of substantial volume molded from thermoplastic resins and the like.

Plastic materials of various compositions are particularly` noted for their shrinkage during the coollng period of fthe molding operation. The problem of shrinkage is of considerable concern in the manufacture of solid "r e' retreated oci. 2o, 1959 ICC the core is formed about the male mold member Without appreciable shrinkage since the core wall portions adjacent thereto are suiciently thin to accomplish this result. However`,when the hot plastic is injected about the core in the second molding operation, the ange portionthereof and the said axially spaced mass thereof are capable of conducting suiicient heat away from the radial core portion adjacent the male mold member to prevent a melting of this corev portion and consequent shrinkage thereof away from the male mold member.

Further objects andadvantages of the present invention'will be apparent from, the following description, reference being had to the accompanyingdrawings vwherein a preferred embodiment of theinvention is clearly shown. l f

In the drawings:

Fig. l is a cross-sectional View of a core taken along Vthe inner face of identical complementary mold mem.-

bers embodying the present invention.

Fig. 2 is a cross-sectional view of a spherical member formed about .the core in Fig.'` 1 taken at the inner face of identical complementary mold members.

The invention will now be described in terms of a spherical knob having an accurately dimensioned threadedopening therein for receiving a threaded end portion l1 of a shaft-like support member as for example an autoing or the shape of the end portion of the lever or supf porting shaft-like member, the plastic material on cooling shrinks away from the top of the mold core in a manner similar `to the formation of pipe in the molding of metal'ingots with the result that the nished article has a deep void at the base of the support receiving opening. As a consequence, when the end portion of the article supporting member is positioned within the opening of the article, vit does not bottom solidly in a predetermined position and may not iirmly be secured in this position by convenient means as for example by screw threads.

Broadly, among the objects of the invention is to provide an article of the type described having an opening therein of predetermined dimensions adapted'for snugly receiving the end of a support member and to provide a method of molding such an article wherein the above described diculty is eliminated.

These and other objects are accomplished in a twostep molding operation wherein a rst molding step produces a relatively thin inner core about a male mold member or mandrel representing the shape of the opening which is to snugly receive the end portion of a support member, which involves no appreciable shrinkage, and a second molding step involves molding a relatively thick layer about the inner core which represents the major portion of the article whereby a solid article is formed having an accurately dimensioned opening there- 1n.

A particular object of the invention is to provide an inner core which is of a configuration and thickness such that no appreciable shrinkage is involved in the molding thereof and which will not melt in the vicinity of the radial portion thereof adjacent the end of the mold member during the second molding operation. To this end, the radial portion of the inner core adjacent the end of the mold member is made relatively thin and provided with a radially extending ange portion. Further, a radially extending portion of substantial mass is provided axially of the said radial portion of the core having at least the radial dimensions of the said radial portion spaced therefrom and connected thereto by a With this coniiguration,

mobile gear shift lever." However, it will be obvious that the invention' is generally applicable to the molding of any article of substantial mass of various shapes which must -have an accurately dimensioned opening therein.

The invention involves a two-step molding operation.

Referring to Fig. l, the first Vstep involves the molding of an inner core 10 Within a more or less conventional injection mold apparatus consisting of identicalV half moldport'ions of which the lower half 12 isl shown including a sprue portion 14, a cavity 16 and a cylindrical openinlg 18 into which is inserted a threaded male mold member 20 positioned and suitably secured in a predetermined location Within the mold cavity. The core member 10 is molded by injecting a thermoplastic resin such as cellulose acetate butyrate at a pressure of about 3-5 tons per sq. in. of projected area in a relatively hot molten or plastic state through the sprue 14. The core 10 consists of a tubular portion 22 surrounding the threaded axial portion of the male mold member 20 and a radial portion 24 adjacent the end 26 of the male mold member 20. Integrally attached to the radial portion 24 is a relatively thin' radially extending ilange 28, and axially spaced from the radial portion 24 is a relatively thin disc-like portion 30 connected thereto by means of a relatively thin neck portion 32 and extending radially to approximately the radial dimensions of the radial portion 24. It will be observed that except for the neck portion 32, the disc-like member 30 is separated from the radial portion 24 of the core by a relatively thin annular space 34. The tubular portion 22 and the radial portion 24 are of a thickness such that no appreciable shrinkage of these parts are involved in the molding of the core member. The neck portion 32 serves only to support the disclike portion 30 in a predetermined spaced relation with respect to the radial portion 24 of thev core and any suitable arrangement may be employed to accomplish this result.

As is illustrated in Fig. 2, the second molding operation involves a more or less conventional mold apparatus consisting of identical half mold portions of which the lofwer half 36 is shown including a spherical cavity 38, a cylindrical opening 40 for receiving the male mold member 20 of Fig. l and a sprue portion 42 adjacent the cylindrical opening 40. In the second molding operation, the male mold member 20 having the core 10 molded thereon is 4 2,903,941 l y j transferred tothe mold'36' and suitably supported' therein. Molten or flowable plastic is injected into the mold cavity through the sprue whereby a second layer 44 constituting the major portions of the finished article is molded about the coreil to form' a: 'solidispherical knobzhaving: an ac'- curately dimensioned openingf therein'corresponding' to the` shape ofthe mold:v member 20'.

shape of1 the core 10 is of! critical importance' in the formation of a solidarticle ofthe' type described'. The annular 'space 34 i's relatively thin? so that the heat ofthe relatively small amount-of plastic material flowing therein is-l carried' away suicientlyf by'the-ilangeportion 28 and the mass 30 froml the radial portion 24 of the core'so that the-radial portion 24? isnot caused to melt and shrink away from the end 26 of the male mold member.

- The exact dimensionso'f themass 3'0; the flange portion 28 andthe annual-,space124fmay-readily-be determined experimentally inthelight of the cycle time of the molding operations, the type of plastic involved in molding and the size of the article to be. molded. Inmolding an article ofeellulose acetate butyra'te involvingA injection pressures of about 3-5 tons per sq; in. of projected area, the core parts are preferably dimensioned so that in the second molding. operation, the 'molding' pressure may be released as s'oon as the`plasticmate`rial has solidified at' the mold wall surfaces at which time the outer core wall surfaces have softened suiiicie'ntly to cause a fusion between the moldV core outer surfaces and the" outerlayer" 44 without appreciable softening of the core walls so that a shrinkage of the core walls away from the male mold member is prevented.- The resulting article is solid throughout and involves no appreciable shrinkage adjacent either the male imold member 20 or the male mold member 36. The presence of the core mass 50 appears to have the further beneficial result of providing. plastic material at the point where a void would form in a single-step molding process.

The method of the present invention is applicable to a wide varietyof plastic materials such as methylmethacrylate resins, polyethylene, cellulose acetate and the like which may be molded in a iiowable or plastic state under the inliuence of heat.

It is to be understood that within minor limits all synthetic resin materials tend to shrink to some extent in all dimensions on cooling from a plastic state and in the aging thereof. In molding plastic articles of the present invention, the effect of this type of shrinkage is corrected for by proper dimensioning of the mold members in relation to the desired dimensions of the iinished article to allow for the shrinkage.

While the embodiments of the present invention constitute a preferred form, it is to be understood that other forms might be adopted.

What is claimed is as follows:

In a method for molding a ball-like article of substantial volume having an opening therein, the stepscomprising; inserting al'metal mold member into a mold, injection" molding a minor portion of the plastic required for the final article about said mold member rfor forming a relatively thin core about the mold member wherein said core includes an extension having an interlock there'- on, cooling the core upon the mold member and removing the same from the mold, inserting the mold member with' the core thereon into a second and larger mold cavity having the desired iinal configuration, injection molding the major portion of the plastic required' for the completed article about said core for filling the mold and for interlocking with the core, removing the article and mold member from the mold after cooling, and nally removing the mold member from the article.

Morin et al. Dec. 16, 1941 Danielson et al Aug. 26, 1952 

